1. Field of the Invention
The present invention relates to thermoplastic processable mixtures based on polyvinyl chloride (PVC) which are stabilized with at least one metal ion in combination with modified polyvinyl alcohol.
2. Description of the Related Art
Thermoplastic processable mixtures based on PVC have to be stabilized against heat and/or UV-radiation in order to reduce chemical decomposition. Especially useful for this propose are metal ions, for example lead or tin salts or the more eco-friendly calcium or zinc compounds. The metal based stabilizers should have a good dispersibility within the polymer matrix without showing agglomeration or incompatibility during the thermoplastic processing of the PVC-mixture. Furthermore, the stabilizers should not migrate to the surface of the processed material in order to prevent so called “plate-out” or “fish eye” disturbances. Therefore, most stabilizers for PVC contain a plurality of components like polyols, beta diketones, phenolic compounds or inorganic stabilizers. See, for example U.S. Pat. No. 5,143,959.
The use of polyvinyl alcohol (PVA) in metal-based stabilizer mixtures for PVC is known. For example, U.S. Pat. No. 5,118,741 discloses the use of partially hydrolyzed PVA during suspension polymerization of PVC in an amount of 0.5 phr at most and the stabilisation of the thus obtained PVC-mixture with metal ions. A co-stabilizing effect of the PVA is not mentioned in this publication. Furthermore, the amounts of PVA added are too low to expect any co-stabilizing effect.
DE 272 8 862 discloses a mixture of PVC, and Ca/Zn salts of a β-diketone with PVA as a polyol compound without giving any details of the PVA used.
The thermal stability of PVC-mixtures containing PVA as co-stabilizer was further analysed by Ikeda et al. al. in “Polymers and Polymer Composites”, p 649 to 662, vol. 11 no. 8, 2003. This paper shows that non-modified PVA is a suitable co-stabilizer for Zn-stabilized PVC mixtures.
The known non-modified PVA-containing stabilizer mixtures for PVC still have deficiencies in view of thermal stability and/or processability during the extrusion of PVC-mixtures.